Pleiotropic effects of cAMP on germination, antibiotic biosynthesis and morphological development in Streptomyces coelicolor

In wild-type Streptomyces coelicolor MT1110 cultures, cyclic adenosine 3′,5′ monophosphate (cAMP) was synthesized throughout the developmental programme with peaks of accumulation both during germination and later when aerial mycelium and actinorhodin were being produced. Construction and characterization of an adenylate cyclase disruption mutant (BZ1) demonstrated that cAMP facilitated these developmental processes. Although pulse-labelling experiments showed that a similar germination process was initiated in BZ1 and MT1110, germ-tube emergence was severely delayed in BZ1 and never occurred in more than 85% of the spores. Studies of growth and development on solid glucose minimal medium (SMMS, buffered or unbuffered) showed that MT1110 and BZ1 produced acid during the first rapid growth phase, which generated substrate mycelium. Thereafter, on unbuffered SMMS, only MT1110 resumed growth and produced aerial mycelium by switching to an alternative metabolism that neutralized its medium, probably by reincorporating and metabolizing extracellular acids. BZ1 was not able to neutralize its medium or produce aerial mycelium on unbuffered SMMS; these defects were suppressed by high concentrations (>1 mM) of cAMP during early growth or on buffered medium. Other developmental mutants (bldA, bldB, bldC, bldD, bldG) also irreversibly acidified this medium. However, these bald mutants were not suppressed by exogenous cAMP or neutralizing buffer. BZ1 also differentiated when it was cultured in close proximity to MT1110, a property observed in cross-feeding experiments between bald mutants and commonly thought to reflect diffusion of a discrete positively acting signalling molecule. In this case, MT1110 generated a more neutral pH environment that allowed BZ1 to reinitiate growth and form aerial mycelium. The fact that actinorhodin synthesis could be induced by concentrations of cAMP (< 20 μM) found in the medium of MT1110 cultures, suggested that it may serve as a diffusible signalling molecule to co-ordinate antibiotic biosynthesis.     

Cardiolipin-deficient cells depend on anaplerotic pathways to ameliorate defective TCA cycle function

Previous studies have shown that the cardiolipin (CL)-deficient yeast mutant, crd1Δ, has decreased levels of acetyl-CoA and decreased activities of the TCA cycle enzymes aconitase and succinate dehydrogenase. These biochemical phenotypes are expected to lead to defective TCA cycle function. In this study, we report that signaling and anaplerotic metabolic pathways that supplement defects in the TCA cycle are essential in crd1Δ mutant cells. The crd1Δ mutant is synthetically lethal with mutants in the TCA cycle, retrograde (RTG) pathway, glyoxylate cycle, and pyruvate carboxylase 1. Glutamate levels were decreased, and the mutant exhibited glutamate auxotrophy. Glyoxylate cycle genes were up-regulated, and the levels of glyoxylate metabolites succinate and citrate were increased in crd1Δ. Import of acetyl-CoA from the cytosol into mitochondria is essential in crd1Δ, as deletion of the carnitine-acetylcarnitine translocase led to lethality in the CL mutant. β-oxidation was functional in the mutant, and oleate supplementation rescued growth defects. These findings suggest that TCA cycle deficiency caused by the absence of CL necessitates activation of anaplerotic pathways to replenish acetyl-CoA and TCA cycle intermediates. Implications for Barth syndrome, a genetic disorder of CL metabolism, are discussed.     

Inactivation of the tricarboxylic acid cycle aconitase gene from Streptomyces viridochromogenes Tü494 impairs morphological and physiological differentiation

The tricarboxylic acid (TCA) cycle aconitase gene acnA from Streptomyces viridochromogenes Tü494 was cloned and analyzed. AcnA catalyzes the isomerization of citrate to isocitrate in the TCA cycle, as indicated by the ability of acnA to complement the aconitase-deficient Escherichia coli mutant JRG3259. An acnA mutant was unable to develop aerial mycelium and to sporulate, resulting in a bald phenotype. Furthermore, the mutant did not produce the antibiotic phosphinothricin tripeptide, demonstrating that AcnA also affects physiological differentiation.     

In vitro mutagenesis of Escherichia coli citrate synthase to clarify the locations of ligand binding sites

In vitro mutagenesis techniques have been used to investigate two structure-function questions relating to the allosteric citrate synthase of Escherichia coli. The first question concerns the binding site of alpha-keto-glutarate, which is a structural analogue of the substrate oxaloacetate and yet has been suggested to be an allosteric inhibitor of the enzyme. Using oligonucleotide-directed mutagenesis of the cloned E. coli citrate synthase gene, we prepared missense mutants, designated CS226H----Q and CS229H----Q, in which histidine residues at positions 226 and 229, respectively, were replaced by glutamine. In the homologous pig heart citrate synthase it is known (Wiegand, G., and Remington, S. J. (1986) Annu. Rev. Biophys. Biophys. Chem. 15, 97-117) that the equivalent of His-229 helps to bind oxaloacetate, while the equivalent of His-226 is nearby. Kinetic and ligand binding measurements showed that CS226H----Q had a reduced affinity for oxaloacetate and alpha-ketoglutarate, while CS229H----Q bound oxaloacetate even less effectively, and was not inhibited by alpha-ketoglutarate at all under our conditions. This parallel loss of binding affinities for oxaloacetate and alpha-ketoglutarate, in two mutants altered in residues at the active site of E. coli citrate synthase, strongly suggests that inhibition of this enzyme by alpha-ketoglutarate is not allosteric but occurs by competitive inhibition at the active site. The second question investigated was whether the known inhibition by acetyl-CoA of binding of NADH, an allosteric inhibitor of E. coli citrate synthase, occurs heterotropically, as an indirect result of acetyl-CoA binding at the active site, or directly, by competition at the allosteric NADH binding site. Using existing restriction sites in the cloned E. coli citrate synthase gene, we prepared a deletion mutant which lacked 24 amino acids near what is predicted to the acetyl-CoA-binding portion of the active site. The mutant protein was inactive, and acetyl-CoA did not bind to the active site but still inhibited NADH binding. Thus acetyl-CoA can interact with both the allosteric and the active sites of this enzyme.     

Engineered citrate synthase alters Acetate Accumulation in Escherichia coli

Metabolic engineering is used to improve titers, yields and generation rates for biochemical products in host microbes such as Escherichia coli. A wide range of biochemicals are derived from the central carbon metabolite acetyl-CoA, and the largest native drain of acetyl-CoA in most microbes including E. coli is entry into the tricarboxylic acid (TCA) cycle via citrate synthase (coded by the gltA gene). Since the pathway to any biochemical derived from acetyl-CoA must ultimately compete with citrate synthase, a reduction in citrate synthase activity should facilitate the increased formation of products derived from acetyl-CoA. To test this hypothesis, we integrated into E. coli C ΔpoxB twenty-eight citrate synthase variants having specific point mutations that were anticipated to reduce citrate synthase activity. These variants were assessed in shake flasks for growth and the production of acetate, a model product derived from acetyl-CoA. Mutations in citrate synthase at residues W260, A267 and V361 resulted in the greatest acetate yields (approximately 0.24 g/g glucose) compared to the native citrate synthase (0.05 g/g). These variants were further examined in controlled batch and continuous processes. The results provide important insights on improving the production of compounds derived from acetyl-CoA.     

Poly-β-hydroxybutyrate (PHB) biosynthesis,tricarboxylic acid activity and PHB content in chickpea (<Emphasis Type="Italic">Cicer arietinum</Emphasis> L.) root nodule

An experiment was conducted to assess the relationship between poly-β-hydroxybutyrate (PHB) biosynthesis and tricarboxylic acid (TCA) activity in desi and kabuli chickpea (Cicer arietinum L.) genotypes. The specific activities of enzymes of PHB metabolism viz., β-ketothiolase (PHB-A), acetoacetyl coenzyme A reductase (PHB-B) and PHB synthase (PHB-C), and those of tricarboxylic acid cycle (citrate synthase (CS) and malate dehydrogenase (MDH) under symbiosis were measured in bacteroids and compared with the PHB accumulation in the nodule and the root. The significant positive correlation was observed between shoot and nodule mass and PHB-A, PHB-B, and PHB-C activities. However, nodule and shoot weights were not significantly correlated with PHB content either in the roots or nodules. The same was true for PHB levels and citrate synthase activity. MDH activity showed a significant negative correlation with nodule PHB. A marked variation and an age dependant increase in malate dehydrogenase activity were measured. A higher capacity for malate oxidation by an increased MDH is likely alter the balance between malate decarboxylation and oxidation, resulting in a higher steady-state concentration of oxaloacetate and that may favor the utilization of acetyl-CoA in the TCA cycle rather than for the synthesis of PHB.     

Differential energetic metabolism during Trypanosoma cruzi differentiation. I. Citrate synthase, NADP-isocitrate dehydrogenase, and succinate dehydrogenase

The activities of the mitochondrial enzymes citrate synthase (citrate oxaloacetatelyase, EC 4.1.3.7), NADP-linked isocitrate dehydrogenase (threo-Ds-isocitrate:NADP+ oxidoreductase (decarboxylating), EC 1.1.1.42), and succinate dehydrogenase (succinate: FAD oxidoreductase, EC 1.3.99.1) as well as their kinetic behavior in the two developmental forms of Trypanosoma cruzi at insect vector stage, epimastigotes and infective metacyclic trypomastigotes, were studied. The results presented in this work clearly demonstrate a higher mitochondrial metabolism in the metacyclic forms as is shown by the extraordinary enhanced activities of metacyclic citrate synthase, isocitrate dehydrogenase, and succinate dehydrogenase. In epimastigotes, the specific activities of citrate synthase at variable concentrations of oxalacetate and acetyl-CoA were 24.6 and 26.6 mU/mg of protein, respectively, and the Michaelis constants were 7.88 and 6.84 microM for both substrates. The metacyclic enzyme exhibited the following kinetic parameters: a specific activity of 228.4 mU/mg and Km of 3.18 microM for oxalacetate and 248.5 mU/mg and 2.75 microM, respectively, for acetyl-CoA. NADP-linked isocitrate dehydrogenase specific activities for epimastigotes and metacyclics were 110.2 and 210.3 mU/mg, whereas the apparent Km's were 47.9 and 12.5 microM, respectively. No activity for the NAD-dependent isozyme was found in any form of T. cruzi differentiation. The particulated succinate dehydrogenase showed specific activities of 8.2 and 39.1 mU/mg for epimastigotes and metacyclic trypomastigotes, respectively, although no significant changes in the Km (0.46 and 0.48 mM) were found. The cellular role and the molecular mechanism that probably take place during this significant shift in the mitochondrial metabolism during the T. cruzi differentiation have been discussed.     

The sensor kinase CitA (DpiB) of Escherichia coli functions as a high-affinity citrate receptor

For the CitA-CitB (DpiB-DpiA) two-component signal transduction system from Escherichia coli, three diverse functions have been reported: induction of the citrate fermentation genes citCDEFXGT, repression of the regulator gene appY, and destabilization of the inheritance of iteron-containing plasmids such as pSC101. This poses the question of the principal biological role of this system. Here it is shown that the periplasmic domain of the E. coli sensor kinase CitA functions as a high-affinity citrate receptor. Two CitA derivatives were purified by affinity chromatography and subjected to binding studies using isothermal titration calorimetry (ITC). One of them, termed CitA215MBP, comprised the N-terminal part of CitA (amino acid residues 1-215), including the two transmembrane helices, and was fused to the amino terminus of the E. coli maltose-binding protein lacking its signal peptide. The second CitA derivative, designated CitAP(Ec), encompassed only the periplasmic domain (amino acid residues 38-177). CitA215MBP bound citrate at 25 degrees C with a K(d) of 0.3 microM and a binding stoichiometry of up to 0.9 in 50 mM sodium phosphate buffer, pH 7. Binding was driven by the enthalpy change (Delta H of -95.7 kJ mol(-1)), whereas the entropy change was not favorable for binding ( T Delta S of -58.6 kJ mol(-1)). ITC experiments with CitAP(Ec) yielded similar K(d) values for citrate (0.15-1.0 microM). Besides citrate, also isocitrate ( K(d) approximately tricarballylate ( K(d) approximately t not malate were bound by CitAP(Ec). The results favor the assumption that the primary biological function of the CitA-CitB system is the regulation of the citrate fermentation genes.     

Disruption of the peroxisomal citrate synthase CshA affects cell growth and multicellular development in Dictyostelium discoideum

Non-mitochondrial citrate synthase catalyses citrate synthesis in the glyoxylate cycle in gluconeogenesis. Screening Dictyostelium discoideum mutants generated by insertional mutagenesis isolated a poor-growing mutant that displayed aberrant developmental morphology on bacterial lawns. Axenically grown mutants developed normally and formed mature fruiting bodies on buffered agar. The affected locus encoded a novel protein (CshA) that was homologous to glyoxysomal citrate synthase. cshA was expressed maximally during vegetative growth and gradually decreased through subsequent developmental stages. An in vitro citrate synthase assay revealed that cshA disruption resulted in a 50% reduction in enzyme activity, implicating CshA as an active citrate synthase. The amino-terminus of CshA was found to have an atypical mitochondrial targeting signal, instead containing a unique nonapeptide sequence (RINILANHL) that was homologous to the conserved peroxisomal targeting signal 2 (PTS2). CshA protein was shown to be localized in the peroxisomes, and the RINILANHL sequence only efficiently targeted the peroxisomal green fluorescent protein. The growth defect of cshA(-) cells was associated with the impairment of phagocytosis and fluid-phase endocytosis, independent from cytokinesis. Disrupted multicellular development on bacterial lawns resulted from the abnormal susceptibility to the environmental conditions, perhaps because of citrate insufficiency. Taken together, these results provide new insights into the function of peroxisomal citrate synthase in cell growth and multicellular development.     

Directing vanillin production from ferulic acid by increased acetyl-CoA consumption in recombinant Escherichia coli

The amplification of gltA gene encoding citrate synthase of TCA cycle was required for the efficient conversion of acetyl-CoA, generated during vanillin production from ferulic acid, to CoA, which is essential for vanillin production. Vanillin of 1.98 g/L was produced from the E. coli DH5alpha (pTAHEF-gltA) with gltA amplification in 48 h of culture at 3.0 g/L of ferulic acid, which was about twofold higher than the vanillin production of 0.91 g/L obtained by the E. coli DH5alpha (pTAHEF) without gltA amplification. The icdA gene encoding isocitrate dehydrogenase of TCA cycle was deleted to make the vanillin producing E. coli utilize glyoxylate bypass which enables more efficient conversion of acetyl-CoA to CoA in comparison with TCA cycle. The production of vanillin by the icdA null mutant of E. coli BW25113 harboring pTAHEF was enhanced by 2.6 times. The gltA amplification of the glyoxylate bypass in the icdA null mutant remarkably increased the production rate of vanillin with a little increase in the amount of vanillin production. The real synergistic effect of gltA amplification and icdA deletion was observed with use of XAD-2 resin reducing the toxicity of vanillin produced during culture. Vanillin of 5.14 g/L was produced in 24 h of the culture with molar conversion yield of 86.6%, which is the highest so far in vanillin production from ferulic acid using recombinant E. coli.     

Probing the Sinorhizobium meliloti-alfalfa symbiosis using temperature-sensitive and impaired-function citrate synthase mutants

To study the role of the decarboxylating leg of the bacterial TCA cycle in symbiotic nitrogen fixation, we used DNA shuffling and localized random polymerase chain reaction mutagenesis to construct a series of temperature-sensitive and impaired-function mutants in the Sinorhizobium meliloti Rm104A14 citrate synthase (gltA) gene. Reducing citrate synthase (CS) activity by mutation led to a corresponding decrease in the free-living growth rate; however, alfalfa plants formed fully effective nodules when infected with mutants having CS activities as low as 7% of the wild-type strain. Mutants with approximately 3% of normal CS activity formed nodules with lower nitrogenase activity and a mutant with less than 0.5% of normal CS activity formed Fix- nodules. Two temperature-sensitive (ts) mutants grew at a permissive temperature (25 degrees C) with 3% of wild-type CS activities but were unable to grow on minimal medium at 30 degrees C. Alfalfa plants that were inoculated with the ts mutants and grown with a root temperature of 20 degrees C formed functional nodules with nitrogenase activities approximately 20% of the wild type. When the roots of plants infected with the ts mutants were transferred to 30 degrees C, the nodules lost the ability to fix nitrogen over several days. Microscopic examination of these nodules revealed the loss of bacteroids and senescence, indicating that CS activity was essential for nodule maintenance.     

Possible role of acetyl-CoA-carboxylase in biosynthesis of mevalonic acid and sterols in rat liver]

Effect of citrate on acetyl-CoA incorporation into mevalonic acid, sterols and fatty acids after preliminary incubation of rat liver extracts under conditions optimal for acetyl-CoA carboxylase activation, was studied. 30 min preincubation with the citrate at 37 degrees C results in a 2--3-fold stimulation of the mevalonic acid biosynthesis from acetyl-CoA in the microsomal and soluble (140 000 g) fraction, and in that of sterols precipitated by digitonin or isolated by TLC in the mitochondria--free fraction. 2-14C-malonyl-CoA incorporation into the mevalonic acid and sterols and biosynthesis of sterols from 2-14C-mevalonic acid were not stimulated under those conditions. A correlation was shown to exist between the activity of acetyl-CoA carboxylase and the rate of acetyl-CoA incorporation into mevalonate and sterols; the activity of beta-hydroxy-beta-methylglutaryl-CoA reductase, limiting the rate of the sterol biosynthesis, was not changed. The stimulating effect of citrate was found to depend on the concentration of acetyl-CoA and NADPH in the medium. The data obtained suggest that the mevalonic acid biosynthesis in rat liver may occur in the presence of acetyl-CoA carboxylase through the formation of malonyl-CoA.     

The periplasmic domain of the histidine autokinase CitA functions as a highly specific citrate receptor.

The two-component regulatory system CitA/CitB is essential for induction of the citrate fermentation genes in Klebsiella pneumoniae. CitA represents a membrane-bound sensor kinase consisting of a periplasmic domain flanked by two transmembrane helices, a linker domain and the conserved kinase or transmitter domain. A fusion protein (MalE-CitAC) composed of the maltose-binding protein and the CitA kinase domain (amino acids 327-547) showed constitutive autokinase activity and transferred the gamma-phosphate group of ATP to its cognate response regulator CitB. The autokinase activity of CitA was abolished by an H350L exchange, and phosphorylation of CitB was inhibited by a D56N exchange, indicating that H-350 and D-56 represent the phosphorylation sites of CitA and CitB respectively. In the presence of ATP, CitB-D56N formed a stable complex with MalE-CitAC. To analyse the sensory properties of CitA, the periplasmic domain (amino acids 45-176) was overproduced as a soluble, cytoplasmic protein with a C-terminally attached histidine tag (CitAPHis). Purified CitAPHis bound citrate, but none of the other tri- and dicarboxylates tested, with high affinity (KD approximately 5 microM at pH 7) in a 1:1 stoichiometry. As shown by isothermal titration calorimetry, the binding reaction was driven by the enthalpy change (DeltaH = -76.3 kJ mol-1), whereas the entropy change was opposed (-TDeltaS = + 46.3 kJ mol-1). The pH dependency of the binding reaction indicated that the dianionic form H-citrate2- is the citrate species recognized by CitAPHis. In the presence of Mg2+ ions, the dissociation constant increased significantly, suggesting that the Mg-citrate complex is not bound by CitAPHis. This work defines the periplasmic domain of CitA as a highly specific citrate receptor and elucidates the binding characteristics of CitAPHis.     

Purification and characterization of 3-ketoacyl-acyl carrier protein synthase III from spinach. A condensing enzyme utilizing acetyl-coenzyme A to initiate fatty acid synthesis.

The 3-ketoacyl-acyl carrier protein (ACP) synthase III from spinach was purified to homogeneity by an eight-step procedure that included an ACP-affinity column. The size of the native enzyme was M(r) = 63,000 based on gel filtration, and its subunit size was M(r) = 40,500 based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggesting that 3-ketoacyl-ACP synthase III may be a homodimer. The purified enzyme was highly specific for acetyl-CoA and malonyl-ACP. The Km for acetyl-CoA was 5 microM when assayed in the presence of 10 microM malonyl-CoA. Acetyl-, butyryl-, and hexanoyl-ACP would not substitute for acetyl-CoA as substrates. The specificity for acetyl-CoA suggested that the physiological function of 3-ketoacyl-ACP synthase is to catalyze the initial condensation reaction in fatty acid biosynthesis. The homogeneous 3-ketoacyl-ACP synthase was capable of catalyzing acetyl-CoA:ACP transacylation but at a rate about 90-fold slower than the condensation reaction with malonyl-ACP. The 3-ketoacyl-ACP synthase was inhibited 100% by 5 mM N-ethylmaleimide or 20 mM sodium arsenite.     

Enzymes involved in fatty acid and polyketide biosynthesis in Streptomyces glaucescens: role of FabH and FabD and their acyl carrier protein specificity

Malonyl acyl carrier protein (ACP) is used as an extender unit in each of the elongation steps catalyzed by the type II dissociated fatty acid synthase (FAS) and polyketide synthase (PKS) of Streptomyces glaucescens. Initiation of straight-chain fatty acid biosynthesis by the type II FAS involves a direct condensation of acetyl-CoA with this malonyl-ACP to generate a 3-ketobutyryl-ACP product and is catalyzed by FabH. In vitro experiments with a reconstituted type II PKS system in the absence of FabH have previously shown that the acetyl-ACP (generated by decarboxylation of malonyl-ACP), not acetyl-CoA, is used to initiate tetracenomycin C (TCM C) biosynthesis. We have shown that sgFabH activity is present in S. glaucescens fermentations during TCM C production, suggesting that it could contribute to initiation of TCM C biosynthesis in vivo. Isotope incorporation studies with [CD3]acetate and [13CD3]acetate demonstrated significant intact retention of three deuteriums into the starter unit of palmitate and complete washout of deuterium label into the starter unit of TCM C. These observations provide evidence that acetyl-CoA is not used directly as a starter unit for TCM C biosynthesis in vivo and argue against an involvement of FabH in this process. Consistent with this conclusion, assays of the purified recombinant sgFabH with acetyl-CoA demonstrated activity using malonyl-ACP generated from either FabC (the S. glaucescens FAS ACP) (k(cat) 42.2 min(-1), K(m) 4.5 +/- 0.3 microM) or AcpP (the E. coli FAS ACP) (k(cat) 7.5 min(-1), K(m) 6.3 +/- 0.3 microM) but not TcmM (the S. glaucescens PKS ACP). In contrast, the sgFabD which catalyzes conversion of malonyl-CoA to malonyl-ACP for fatty acid biosynthesis was shown to be active with TcmM (k(cat) 150 min(-1), K(m) 12.2 +/- 1.2 microM), AcpP (k(cat) 141 min(-1), K(m) 13.2 +/- 1.6 microM), and FabC (k(cat) 560 min(-1), K(m) 12.7 +/- 2.6 microM). This enzyme was shown to be present during TCM C production and could play a role in generating malonyl-ACP for both processes. Previous demonstrations that the purified PKS ACPs catalyze self-malonylation and that a FabD activity is not required for polyketide biosynthesis are shown to be an artifact of the expression and purification protocols. The relaxed ACP specificity of FabD and the lack of a clear alternative are consistent with a role of FabD in providing malonyl-ACP precursors for PKS as well as FAS processes. In contrast, the ACP specificity of FabH, isotope labeling studies, and a demonstrated alternative mechanism for initiation of the PKS process provide unequivocal evidence that FabH is involved only in the FAS process.